A telephone system generally has a telephone switching system (Telephone Central Office) or an exchange and one or more subscriber terminals which are connected to the telephone switching system by means of a respective subscriber line. The subscriber terminals can be telephones or other telephone equipment. The subscriber line generally has two conductors or two wires which are referred to as the “tip” (a-wire) and the “ring” (b-wire). The tip wire and the ring wire transport both AC signals and DC signals. The subscriber terminal together with the associated tip and ring wires is generally referred to as a loop or subscriber loop (SL).
The telephone switching system handles the switching of telephone signals between subscriber terminals. Industrial standards, such as Telcordia (Bellcore) Technical Reference TR-NWT-00057, Functional Criteria for Digital Loop Carrier (DLC) Systems, Issue Jan. 2, 1993, determine the electrical signal levels which are used for switching, connecting and signaling within the telephone system. If, by way of example, a telephone call to a specific subscriber terminal is requested, the telephone switching system needs to transmit ringing signals to the telephone terminal in order to signal the incoming call.
The telephone system's telephone switching system normally contains a linecard, a ringing signal generator and a voltage supply for the linecard and the ringing signal generator. The linecard supplies a plurality of subscribers and is normally fitted in the telephone switching system in a rack. For each subscriber, the linecard has a subscriber line interface circuit (SLIC) and a CODEC circuit (coder/decoder circuit).
The subscriber line interface circuit is coupled to a respective subscriber line by means of associated connections on the linecard and is connected to the subscriber by means of the subscriber line. The subscriber line interface circuit is normally connected to the CODEC circuit using four lines, a reception line and a transmission line and two respective associated ground lines. The subscriber line interface circuit (SLIC) couples the analog subscriber line operated at a high voltage to the analog and digital circuits in the CODEC circuit, which are operated at low voltages, and converts the signals transmitted in the four lines connected to the CODEC circuit into a two-wire signal for the subscriber line. The subscriber line interface circuit normally supports the known “BORSHT” functions of battery feed, overvoltage protection, generation of the ringing signal, signaling, coding or PCM-conversion, the conversion between two-wire transmission and four-wire transmission (hybrid) described above, and testing.
The CODEC circuit is normally connected by means of a data transmission line and data reception line to a further connection on the linecard, said further connection being connected to a data line. The data line is used to transmit voice information in the voice band using PCM-coded digital signals and information data, for example using FSK-modulated digital signals. The digital signals are decoded by the CODEC circuit and are converted into analog signals for the subscriber line interface circuit.
The reception line between the CODEC circuit and the subscriber line interface circuit normally has a frequency shift keying (FSK) circuit connected to it. In the case of a received telephone call, for example, the FSK circuit generates the telephone number which is to be displayed on the subscriber terminal. The frequency shift keying used in the FSK circuit is frequency modulation which uses two frequencies, one frequency of the two frequencies representing the digital one, while the other frequency of the two frequencies represents the digital zero.
The details relating to transmission of information data via lines are stipulated, by way of example, in the European Telecommunication Standard ETS with the reference No. 300659-1 (PSTN; Subscriber line protocol over the local loop for display (and related) services; Part 1 On hook transmission) from ETSI. The information data are transmitted in the voice band of a line using the aforementioned FSK modulation. For FSK modulation, the stipulations from ITU-T V.23 for the forward channel at a baud rate of 1200 bauds apply, for example. Depending on the purpose of the information, the information data can be transmitted both in the respective subscriber terminal's call state, i.e. before a signaled, incoming directed call is taken, and in the subscriber terminal's connected state. In terms of a loop resistance of the subscriber loop which contains the subscriber line and the subscriber terminal, the call state is equivalent to the quiescent state. Since, in the quiescent state, the subscriber terminal's receiver is generally seated on the hook, the term “on-hook” is used for this. In the connected state, a subscriber terminal's receiver has been lifted from the hook, and hence ETS stipulates the term “off-hook” for this. In a subscriber loop's on-hook state, information data such as the telephone number of the calling subscriber are transmitted, as mentioned above. The FSK signals which transmit these information data are referred to herein as on-hook data transmission signals.
In DLC (Digital Loop Carrier) systems, the power loss in the on-hook state of the subscriber loop is of great significance, since these systems are frequently powered remotely by supply devices, are positioned independently and need to maintain the telephone service during mains failure, for example (Life Line Support). In the case of conventional linecards and subscriber line interface circuits used in such DLC systems, for example, the power loss is normally accepted in the on-hook state, however, and the subscriber line interface circuit is left in an active operating mode, for example, in the on-hook state.
To reduce the power loss of linecards and subscriber line interface circuits, other known solutions provide a dedicated power-loss reducing on-hook operating mode in the subscriber line interface circuit. However, if a subscriber line interface circuit is operated in this on-hook operating mode, this results in a poorer transmission response when transmitting on-hook data transmission signals, on account of the lower power available for data transmission.